"Market cap" estimates assume that everyone could sell without the price moving. It's going to be far more sensitive to price changes than an actual country's GDP, it's based on what current trading prices are.

And Bitcoin, unlike a company with a huge market cap, isn't providing goods or services. E.g. if Apple disappeared into thin air, everyone hoping to buy new Apple phones or computers would be dramatically affected and that would have a ton of ripple effects. But if Apple's sales dropped to 0 and then they went out of business, their valuation would be down to 0 but a lot of people would've recouped some of their investment on the way down - the people selling earlier being better off than the ones later. This is a crazy scenario, though, since they're sales wouldn't go to 0 overnight.

So who's actually relying on Bitcoin? Some companies in the space, sure. Probably a bunch of individuals who did stuff like use bitcoin as part of their wealth portfolio when taking out loans. But if those people had those holdings wiped out, will the ramifications be huge? My guess is probably not but I don't actually know how many people would, for instance, be unable to pay their mortgage if their crypto holdings imploded.

(This will be interesting to see if a crash does happen: in a world where you believe prices always go up, the "clever" move is to not sell your bitcoin, but to borrow against it, so it appreciates in the background still. But in a world where there's a crash, this could leave you worse off than before, the debt won't go away just because the price fell.)